1. Field of the Invention
The present invention relates to magnetic powder and an isotropic bonded magnet produced using the magnetic powder.
2. Description of the Prior Art
For reduction in size of motors, it is desirable that a magnet has a high magnetic flux density (with the actual permeance) when it is used in the motor. Factors for determining the magnetic flux density of a bonded magnet include magnetic performance (that is, magnetization) of the magnetic powder and the content (that is, compositional ratio) of the magnetic powder in the bonded magnet. Accordingly, when the magnetic performance (magnetization) of the magnetic powder itself is not sufficiently high, a desired magnetic flux density cannot be obtained unless the content of the magnetic powder in the bonded magnet is raised to an extremely high level.
At present, most of practically used high performance rare-earth bonded magnets use the isotropic bonded magnets which are made using MQP-B powder manufactured by MQI Corp. as the rare-earth magnetic powder thereof. The isotropic bonded magnets are superior to the anisotropic bonded magnets in the following respect; namely, in the manufacture of the bonded magnet, the manufacturing process can be simplified because no magnetic field orientation is required, and as a result, the rise in the manufacturing cost can be restrained. On the other hand, however, the conventional isotropic bonded magnets such as those manufactured using MQP-B powder having the following disadvantages.
(1) The conventional isotropic bonded magnets do not have sufficiently high magnetic flux density. Specifically, because of the low magnetic performance (that is, the insufficient magnetization) of the magnetic powder used, the content of the magnetic powder to be contained in the bonded magnet has to be increased. However, the increase in the content of the magnetic powder leads to the deterioration in the moldability of the bonded magnet, so there is a certain limit in this attempt. Moreover, even if the content of the magnetic powder is somehow managed to be increased by changing the molding conditions or the like, there still exists a limit to the obtainable magnetic flux density. For these reasons, it is not possible to reduce the size of the motor by using the conventional isotropic bonded magnets.
(2) Since the conventional bonded magnet has high coercivity (coercive force), magnetizability thereof is poor, thus requiring a relatively high magnetic field of magnetization.
(3) Although there are reports concerning nanocomposite magnets having high remanent magnetic flux densities, their coercive forces, on the contrary, are so small that the magnetic flux densities (for the permeance in the actual use) obtainable for the practical motors are very low. Further, these magnets have poor heat stability due to their small coercive forces.
(4) The conventional bonded magnets have low temperature characteristics (that, is heat resisting property or heat stability). In particular, the irreversible flux loss remarkably drops when the coercive force (HCJ) is lowered.
(5) The bonded magnet has poor corrosion resistance and heat resisting property. In particular, when the content of the magnetic is increased in order to compensate the low magnetic performance of the magnetic powder (that is, when the magnetic flux density of the bonded magnet is extremely raised), the corrosion resistance and heat resisting property remarkably drop.
Therefore, it is necessary to cover the outer surface of the bonded magnet with a coating, especially a resin coating which is capable of obtaining high corrosion resistance property, but this in turn leads to increased manufacturing cost and the presence of the resin layer results in lowered magnetic performance (this makes it difficult for a motor to generate a high torque).
It is therefore an object of the present invention to provide magnetic powder that can produce a magnet having a high magnetic flux density and having excellent magnetizability and reliability especially corrosion resistance property and temperature characteristics (that is, heat resisting property and heat stability), and provide an isotropic bonded magnet formed from the magnetic powder.
In order to achieve the above object, the present invention is directed to magnetic powder having an alloy composition represented by Rx(Fe1-yCoy)100-x-x-wBXSiw (where R is at least one kind of rare-earth element, x is 8.1-9.4 at %, y is 0-0.30, z is 4.6-6.8 at %, and w is 0.2-3.0 at %), the magnetic powder being constituted from a composite structure having a soft magnetic phase and a hard magnetic phase, wherein the magnetic powder has characteristics in which, when the magnetic powder is formed into an isotropic bonded magnet by mixing with a binding resin and then molding it, the irreversible susceptibility ("khgr"irr) which is measured by using an intersectioning point of a demagnetization curve in the J-H diagram representing the magnetic characteristics at the room temperature and a straight line which passes the origin in the J-H diagram and has a gradient (J/H) of xe2x88x923.8xc3x9710xe2x88x926 H/m as a starting point is less than 5.0xc3x9710xe2x88x927 H/m, and the intrinsic coercive force (HCJ) of the magnet at the room temperature is in the range of 406-717 kA/m.
As described above, according to the present invention, the following effects can be obtained.
Since each of the magnetic powders has the composite structure having a soft magnetic phase and a hard magnetic phase and contains a predetermined amount of Si, the magnetic powder exhibits excellent magnetic characteristics so that intrinsic coercive force and rectangularity thereof are especially improved, and excellent corrosion resistance property is exhibited.
The absolute value of the irreversible flux loss is small and excellent heat resisting property (heat stability) can be obtained.
Because of the high magnetic flux density that can be secured by this invention, it is possible to obtain a bonded magnet with high magnetic performance even if it is isotropic. In particular, since magnetic performance equivalent to or better than the conventional isotropic bonded magnet can be obtained with a magnet of smaller volume as compared with the conventional isotropic bonded magnet, it is possible to provide a high performance motor of a smaller size.
Moreover, since a higher magnetic flux density can be secured, in manufacturing a bonded magnet sufficiently high magnetic performance is obtainable without pursuing any means for elevating the density of the bonded magnet. As a result, the dimensional accuracy, mechanical strength, corrosion resistance, heat resisting property (heat stability) and the like can be further improved in addition to the moldability, so that it is possible to readily manufacture a bonded magnet with high reliability.
Since the magnetizability of the magnet according to this invention is excellent, it is possible of magnetize a magnet with a lower magnetizing field. In particular, multipolar magnetization or the like can be accomplished easily and surely, and further a high magnetic flux density can be obtained.
Since the bonded magnet of the present invention does not require to have a high density, the present invention is adapted to the manufacturing method such as the extrusion molding method or the injection molding method by which molding at high density is difficult as compared with the compression molding method, and the effects described in the above can also be obtained in the bonded magnet manufactured by these molding methods. Accordingly, various molding methods can be selectively used and thereby the degree of selection of shapes for the bonded magnet can be expanded.
In this connection, it is preferred that the composite structure is a nanocomposite structure.
Further, in the present invention, it is preferred that said R comprises rare-earth elements mainly containing Nd and/or Pr. In this case, it is preferable that said R includes Pr and its ratio with respect to the total mass of said R is 5-75%. When the ratio lies within this range, it is possible to improve the coercivity and the rectangularity by hardly causing a drop in the remanent magnetic flux density.
Furthermore, in the present invention, it is also preferred that said R includes Dy and its ratio with respect to the total mass of said R is equal to or less than 14%. When the ratio lies within this range, the coercivity can be improved without causing marked drop in the remanent magnetic flux density, and the heat resisting property is also improved.
Moreover, it is preferred that the magnetic powder is obtained by quenching the ally of a molten state.
It is also preferred that the magnetic powder is obtained by pulverizing a quenched ribbon of the alloy which is manufactured by using a cooling roll.
Further, it is also preferred that the magnetic powder is subjected to a heat treatment for at least once during the manufacturing process or after its manufacture.
Furthermore, it is also preferred that the average grain size of the magnetic powder lies in the range of 0.5-150 xcexcm.
The present invention is also directed to an isotropic rare-earth bonded magnet, which is formed by binding the magnetic powder as set forth in the above with a binding resin.
The present invention is also directed to an isotropic rare-earth bonded magnet formed by binding magnetic powder with a binding resin, wherein the isotropic rare-earth bonded magnet is characterized in that the irreversible susceptibility ("khgr"irr) which is measured by using an intersectioning point of a demagnetization curve in the J-H diagram representing the magnetic characteristics at the room temperature and a straight line which passes the origin in the J-H diagram and has a gradient (J/H) of xe2x88x923.8xc3x9710xe2x88x926 H/m as a starting point is equal to or less than 5.0xc3x9710xe2x88x927 H/m, and the intrinsic coercive force (HCJ) of the magnet at the room temperature is in the range of 406-717 kA/m.
In this case, it is preferred that the magnetic powder used in the isotropic bonded magnet includes Si.
Further, it is preferred that said magnetic powder is formed of R-TM-B-Si based alloy (where R is at least one rear-earth element and TM is a transit metal containing Iron as a major component thereof).
Moreover, it is preferred that the magnetic powder has a composite structure having a soft magnetic phase and a hard magnetic phase.
Moreover, it is also preferred that the isotropic bonded magnet is to be subjected to multipolar magnetization or is already subjected to multipolar magnetization. In this case, it is also preferred that the isotropic bonded magnet is used for a motor.
These and other objects, structures and advantages of the present invention will be apparent from the following detailed description of the invention and the examples taken in conjunction with the appended drawings.